For vehicle lighting devices, it is known to use a semiconductor light emitting element such as a LED (Light Emitting Diode), or the like, as a light source. The lighting control circuit for controlling the lighting of the LED is installed into the vehicle lighting device of this type.
In constructing the lighting control circuit, a switching regulator having a switching element and a transformer may be used. Then an input voltage fed from a DC power supply is accumulated in the transformer as electromagnetic energy during an ON-operation of the switching element. Also, electromagnetic energy accumulated in the transformer is supplied to the LED from the secondary side of the transformer via a rectifier diode and smoothing capacitor during an OFF-operation of the switching element (see JP-A-2004-134147 at pages 3 to 6; FIGS. 1 and 2)). In this lighting control circuit, operation of the switching regulator is stopped when failure of a power supply voltage is sensed. Also, operation of the switching regulator is restarted when such failure is no longer sensed.
A microcomputer has been used to generate a switching signal to control the ON/OFF-operation of the switching element. In controlling the switching element in the switching regulator by using the microcomputer, a countermeasure must be devised to prevent a situation in which the electronic parts, such as the switching element and the like, are destroyed as a result of an increase of the primary current of the switching regulator. The LED typically is electrically stable, and the impedance of the LED typically is not changed in a short time, in contrast to an electric-discharge lamp. When the LED or the like is assumed as the load, a variation on the output side of the switching regulator is small. Thus, only a sudden change of the input voltage (power-supply voltage) needs to be considered as the cause of the increase of the primary current. Therefore, attempts have been made to limit the current on the primary side of the switching regulator by monitoring the current flowing through the primary side each time the switching element is turned ON/OFF and then limiting the current on the primary side when the current on the primary side is abnormal.
To limit the current on the primary side of the switching regulator when the input voltage of the switching regulator is changed suddenly by monitoring the current flowing through the primary side each time the switching element is turned ON/OFF, a microcomputer having a response speed of several tens nanoseconds (ns) to several micro-seconds (μn) is needed to sense a change of the input voltage. Therefore, the microcomputer must be driven at 1 GHz, for example, which increases the cost of the lighting control circuit.
Also, in order to limit an increase of the current on the primary side of the switching regulator, a “duty max” (a maximum value of an ON duty) is provided to the switching signal, which is used to turn the switching element ON/OFF, and the ON duty is limited to a duty max when the input voltage is changed abruptly. In this event, if the microcomputer executes not only the process of sensing a sudden change of the input voltage, but also the process of computing the duty max, it takes much time to execute the process. In other words, after the input voltage is changed suddenly, it takes a relatively long time until an increase of the current on the primary side can be limited. Therefore, there is a possibility that the switching element cannot be protected promptly.
If the microcomputer is caused to take in the input voltage and execute either the process of sensing the sudden change of the input voltage or the process of limiting the current on the primary side, a lot of time is required to execute an arithmetic operation. Thus, it is possible that the lighting control circuit cannot respond to the sudden change of the input voltage.
In the event that the input voltage continues to change abruptly, for example if the input voltage is increased twice from 12 V to 24 V, as shown in FIG. 14, a peak value of the current on the primary side of the switching regulator is accumulated sequentially every time a pulse (switching signal) is generated to ON/OFF-operate the switching element. Suppose, for example, that a peak current on the primary side is defined as “ip” in the normal operation and the input voltage is abruptly changed twice at a time t1, the peak current on the primary side is increased as 2ip, 3ip, 4ip, 5ip, 6ip at times t2 to t6 every time the pulse is output. This is because a rising gradient of ip is doubled but a falling gradient is not changed. Also, since one pulse period is 10 μs when a PFW (PWM) frequency used to ON/OFF-operate the switching element is set to 100 kHz, ip is increased 11 times in a period of 100 μs (equivalent to 10 pulses). Therefore, when ip=5 A is set in the normal operation, ip=55 A is obtained after 100 μs. If the current flowing through the switching element cannot be limited until 100 μs has elapsed after the input voltage was changed suddenly, there is a possibility that the switching element will be destroyed.
Also, if the switching element is operated in a current boundary mode, the switching element is not turned ON unless the current on the secondary side of the switching regulator becomes 0. In this event, if about 100 μs passes until the ON duty of the switching element is set after the microcomputer sensed a current boundary, an increase in the current on the primary side of the transformer is caused subsequently to the sudden change of the input voltage before the switching element goes to the operation in the current boundary mode. Thus, it is possible that the switching element will be destroyed.